Alloy for building up valve

ABSTRACT

An alloy for building up a valve according to the present invention comprises: with respect to the total weight of the alloy taken as 100%, 30 to 40% by weight of chromium; 15 to 31% by weight of nickel; 7 to 20% by weight of molybdenum; 0.7 to 2.2% by weight of carbon; 1.5% or less by weight of silicon; and balance of iron and inevitable impurities. The alloy is superior in hardness at high temperatures, PbO resistance and PbO+PbSO 4  resistance. In addition, the alloy is suitable for powder buildup welding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an alloy for building up valves of internalcombustion engines, especially for building up exhaust valves ofautomobile engines.

2. Description of the Prior Art

Many conventional valves of internal combustion engines have been usedafter buildup welding the portions thereof to be abraded with varioussurface-hardening alloys.

A cobalt-base alloy, consisting mainly of cobalt-chromium and smallamounts of tungsten, molybdenum, carbon and so on, is often employed asa buildup alloy for exhaust valves of automobile engines since thecobalt-base alloy is superior in hardness at high temperatures and leadoxide corrosion resistance (hereinafter referred to PbO resistance).However, the cobalt-base alloy contains a large amount of expensivecobalt, and is a highly expensive material for buildup welding.Therefore a less expensive material which exhibits no less performancethan that of the cobalt-base alloy has been required for buildup weldingthe exhaust valves of the automobile engines.

Japanese Unexamined Patent Publication (KOKAI) No. 153872/1984 disclosesan iron-chromium-nickel alloy for engine valves developed for solvingthe above mentioned problem. The iron-chromium-nickel alloy is employedfor forming portions of the engine valve which are required to withstandabrasion by buildup by casting or by building up with a welding rod. Tothe iron-chromium-nickel alloy is added tungsten and titanium forcontrolling and refining the growth of the crystal grains of the basemetal. To the iron-chromium-nickel there is further added manganese fordeoxidizing and desulfurizing. The iron-chromium-nickel alloyfurthermore contains aluminum for improving the IbO resistance. However,the inventors of the present invention have investigated and discoveredthat the iron-chromium-nickel alloy is inferior in its resistance toPbO. Further, instead of the buildup with the welding rod, a method forbuildup with alloy powder has been drawing attention because of itsoperability, buildup efficiency and ease of automation. However, whenthe iron-chromium-nickel alloy is employed for the method for thebuildup with alloy powder, the following drawbacks arise: i.e., a poorpowder deposition rate, an uneven bead configuration, pin holes and plowholes in the buildingup layer and oxide intrusions into the builduplayer. The above drawbacks are believed to result from the titaniumadded for controlling and refining the growth of the crystal grains ofthe base metal and the aluminium for improving the PbO resistance.

Further, Japanese Unexamined Patent Publication (KOKAI) No. 179891/1987discloses a nickel-chromium-molybdenum alloy for buildup, although it isnot so cheap as the abovementioned iron-chromium-nickel alloy. Thenickel-chromium-molybdenum alloy is superior in hardness at hightemperatures and the PbO resistance, and the buildup method with alloypowder can be conducted favorably. However, the inventors haveinvestigated and found that the corrosion of the exhaust valves iscaused not only by lead oxide but also by lead oxide containing leadsulfate. The inventors have discovered that thenickel-chromium-molybdenum alloy is inferior in corrosion resistanceagainst lead oxide containing lead sulfate (hereinafter referred to asPbO+PbSO₄ resistance).

SUMMARY OF THE INVENTION

It is an object of the present invention to develop a buildup weldingalloy applicable to the buildup welding and the buildup method withalloy powder, and the build-up welding alloy maintaining requiredhardness at high temperatures and having excellent PbO resistance andPbO+PbSO₄ resistance, thereby satisfying the requirements of valves ofinternal combustion engines, especially the severe requirements ofvalves of recent high performance automobile engines.

An alloy for building up a valve according to the present inventioncomprises: with respect to the total weight of the alloy taken as 100%,30 to 40% by weight of chromium; 15 to 31% by weight of nickel; 7 to 20%by weight of molybdenum; 0.7 to 2.2% by weight of carbon; 1.5% or lessby weight of silicon; and balance of iron and inevitable impurities.

It is a feature of the present invention that the buildup alloy contains7% or more by weight of molybdenum, more preferably 10% or more byweight of molybdenum in an iron-chromium-nickel alloy. The alloyaccordingly becomes superior in hardness at high temperatures, and bothin PbO resistance and PbSO₄ resistance. Further, the alloy is alsosuitable for the powder buildup method since the alloy does not containa large amount of titanium, aluminum and manganese which are undesirablefor the components of the powder buildup alloy.

The reasons for restricting the contents of the buildup alloy componentsof the present invention as above will be hereinafter explained.

(Chromium)

The chromium is solidified in the iron, nickel and molybdenum to form aprimary crystal solid solution. The primary crystal solid solutionconstitutes the substrate of the alloy. A part of the chromium bondswith the carbon as well as the molybdenum to form composite carbideconsisting mainly of chromium and molybdenum. In the alloy compositionregion of the present invention, the composite carbide forms eutecticwith the solid solution, and is present in the eutectic. Namely, thechromium coexists with the carbon, and improves the hardness of thealloy. Further, the chromium contributes to improve the PbO resistanceand the PbO+PbSO₄ resistance. The PbO resistance is improved by theformation of chromic oxide on the surface of the alloy, and the PbO+PbSOresistance is improved by the formation cf chromic sulfate and chromicoxide on the surface of the alloy, whereby, the corrosion of the alloyhas been prevented from propagating.

The effect of the chromium addition becomes poor when the chromiumcontent is 30% or less by weight. When the chromium content exceedsequal 40% or more by weight, the effect of the chromium addition has notbeen enhanced. In addition, the ductility of the alloy tends todeteriorate when the chromium content is exceeds 40% or more by weight.

The chromium content is accordingly restricted in the range of 30 to 40%by weight.

(Nickel)

The nickel is solidified in the solid solution of the substrate toimprove the tenacity of the alloy and maintain the hardness thereof athigh temperatures. The relationship between the nickel content and thevickers hardness is shown in FIG. 1. Further, the nickel coexists withthe chromium to improve the PbO resistance. Namely, the chromic oxidefilm becomes finer and bonds more firmly on the alloy by the nickel.

The relationship between the nickel content and the PbO resistance andthe PbO+PbSO₄ resistance, namely the relationship between the nickelcontent and the corrosion loss is shown in FIG. 2. The PbO resistanceimprovement effect of the nickel addition becomes poor when the nickelcontent is 15% or less by weight. When the nickel content exceeds 31% ormore by weight, the effect of the nickel addition cannot be expected toimprove. On the contrary, a high nickel content tends to deteriorate thePbO+PbSO₄ resistance. This is because the nickel sulfate of a lowmelting point is generated on the surface of the alloy, and thecorrosion of the alloy is facilitated. Hence, it is undesirable to add alarge amount of nickel in the alloy.

The nickel content is accordingly restricted in the range of 15 to 31%by weight.

(Molybdenum)

The molybdenum is solidified in the solid solution of the substrate toimprove the tenacity of the alloy as well as the strength of the alloy.Further, the molybdenum bonds with the carbon as well as the chromiumand forms the composite carbide of the chromium and molybdenum toimprove the hardness of the alloy at high temperatures. Furthermore, themolybdenum contributes greatly to the improvement of the PbO resistance,especially to the improvement of the PbO+PbSO₄ resistance. Themolybdenum sulfate as well as the chromic oxide and chromic sulfate areformed on the surface of the alloy, whereby the corrosion of the alloyhas been prevented from propagating.

The alloy exhibits the most excellent PbO+PbSO₄ resistance when themolybdenum content is 10% by weight. When the molybdenum content is lessthan 10% by weight, the PbO+PbSO₄ resistance tends to deteriorate. Alimitation of the molybdenum content lies at approximately 7% by weight,though it depends on the nickel content. The PbO+PbSO₄ resistancedeteriorates when the molybdenum content is 7% or less by weight. It isalso undesirable to add the molybdenum by 20% or more by weight, sincethe alloy becomes too hard and the ductility thereof deteriorates.

The molybdenum content is accordingly restricted in the range of 7 to20% by weight.

(Carbon)

A part of the carbon is solidified in the solid solution consistingmainly of the iron-nickel as the substrate to increase the strength andhardness of the substrate, but most of the carbon forms the compositecarbide consisting mainly of the chromium and molybdenum and formseutectic composite carbide with the solid solution of the substrate. Asa result, the carbon increases the hardness and abrasion resistance ofthe alloy, and decreases the melting point of the alloy.

The effect of the carbon addition is poor when the carbon content is0.7% or less by weight. When the carbon content exceeds 2.2% by weight,the composite carbide is formed too much, and hyper-eutectic appears tocrystallize the primary crystal of the composite carbide consistingmainly of the chromium and molybdenum. Consequently, it is undesirableto add the carbon by more than 2.2% by weight, since the ductility ofthe alloy deteriorates.

The carbon content is accordingly restricted in the rang of 0.7 to 2.2%by weight.

(Silicon)

Silicon is usually added as a deoxidizer. In the alloy of the presentinvention, however, when the silicon is added by 1.5% or more by weight,the tenacity of the alloy deteriorates. This is because an eutecticstructures of a low melting point is formed between the solid solutionof the substrate, consisting mainly of iron-nickel, and molybdenumsilicide.

The silicon content is accordingly restricted to 1.5% or less by weight.

(Inevitable impurities)

The alloy of the present invention can be employed for the powderbuildup. In this case, however, there arises concerns on thecharacteristics of the powder buildup alloy; i.e., the deposition rateduring the powder buildup, the bead configuration, the defects in thebuildup layer and the like.

The inventors of the present invention have investigated variousinevitable impurities contained in the alloy of the present invention,and have discovered that it is preferable to keep the {O} content,resulting from the surface oxidation of the powder and the like, to 0.2%or less by weight. The inventors have also discovered that it ispreferable to keep the contents of manganese, aluminium, and titanium,which work as deoxidizers, to 0.1% or less by weight, respectively. Whenthe {O} content is more than 0.2% by weight, and when the contents ofthe manganese, aluminium and titanium are more than 0.1% by weight,respectively, the bead configuration becomes uneven, and defects mayoccur in the buildup layer.

Further, the protective film of the chromium oxide, formed on thesurface of the alloy and contributing to the PbO resistance, is affectedwhen active metals such as the manganese, aluminium and titanium arecontained 0.1% or more by weight in the alloy. In this case, theprotective film of the chromium oxide turns into a protective filmthereof containing the oxides of manganese, aluminium and titanium. Theprotective film becomes inferior in the finess and adhesion to thealloy. The PbO resistance of the alloy thus deteriorates since theprotective film does not exhibits the sufficient protection effect.

It is accordingly preferable to restrict the contents of the manganese,aluminium and titanium, contained as inevitable impurities in the alloy,to 0.1% or less by weight to obtain the alloy having superior PbOresistance and suitable for the powder buildup.

As described above, the alloy for building up a valve of the presentinvention is less expensive, and is superior in the hardness at hightemperatures, the PbO resistance and the PbO+PbSO₄ resistance. Further,the alloy is suitable for the powder buildup. When an exhaust valve ismade by the powder buildup employing the alloy of the present invention,the exhaust valve endures the severe operation condition of the highperformance automobile engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of this inventionwill become fully apparent from the following description taken inconjunction with the accompanying drawings, in which;

FIG. 1 is a graph showing the relationship between the nickel contentand the Vickers hardness at 700° C.; and

FIG. 2 is a graph showing the relationship between the nickel contentand the corrosion losses in PbO resistance test and PbO+PbSO₄ resistancetest.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Having generally described this invention, a further understanding canbe obtained by reference to certain specific preferred embodiments whichare provided herein for purposes of illustration only and are notintended to be limiting unless otherwise specified.

The hardness at an ordinary temperature, the hardness at a hightemperature, the PbO resistance, the PbO+PbSO₄ resistance and the powderbuildup characteristic were examined to evaluate the alloys of preferredembodiments and comparative examples. The alloys of the preferredembodiments and comparative examples were prepared by mixing and meltingthe components in the compositions shown in Table 1 and 2 in theordinary manner. The results of the test concerning the preferredembodiments and comparative examples are shown in Table 1 and Table 2,respectively. The data of the preferred embodiment alloy Nos. 3, 5, 6,16, and 17, and comparative example alloy Nos. 4 and 5 in the hardnesstest at high temperature, the PbO resistance test, and the PbO+PbSO₄resistance test, set forth in Tables 1 and 2, are plotted in FIGS. 1 and2, respectively, to explain the relationship between the nickel contentand the hardness at high temperature, and the relationship between thenickel content and the PbO resistance and the PbO+PtSO₄ resistance,respectively. The methods for measuring and examining the abovecharacteristics will be described hereinafter.

                                      TABLE 1                                     __________________________________________________________________________                                        Hardness        PbO+                      No. of                              at   Hardness                                                                           PbO   PbSO.sub.4                                                                         Pow                  Alloys                              Ordi-                                                                              at   resis-                                                                              resis-                                                                             Bui                  (Present                            nary High tance tance                                                                              Cha                  Inven-                                                                             Composition (weight %)         temp.                                                                              temp.                                                                              mg/cm.sup.2                                                                         mg/cm.sup.2                                                                        act                  tion)                                                                              Cr Ni Mo C Si                                                                              Fe   Co                                                                              W Mn Al Ti Hv   Hv   per hr                                                                              per                                                                                ist                  __________________________________________________________________________     1   30.1                                                                             15.3                                                                              7.2                                                                             2.0                                                                             1.5                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 520  341  75    88   o                     2   30.5                                                                             20.8                                                                             10.5                                                                             1.1                                                                             0.3                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 410  310  78    81   o                     3   35.4                                                                             20.0                                                                             10.2                                                                             1.3                                                                             0.2                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 450  315  62    67   o                     4   37.8                                                                             20.5                                                                             10.3                                                                             1.2                                                                             0.3                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 560  330  38    42   o                     5   35.3                                                                             15.2                                                                             10.8                                                                             1.3                                                                             0.3                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 520  323  75    70   o                     6   35.5                                                                             25.0                                                                             11.2                                                                             1.1                                                                             0.4                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 440  315  83    71   o                     7   35.0                                                                             20.3                                                                              7.0                                                                             1.5                                                                             0.4                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 390  305  55    92   o                     8   35.4                                                                             20.8                                                                             15.2                                                                             1.3                                                                             0.2                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 530  328  68    66   o                     9   33.3                                                                             24.8                                                                             20.0                                                                             0.7                                                                             0.1                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 560  365  70    59   o                    10   35.8                                                                             21.0                                                                             10.5                                                                             1.5                                                                             0.1                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 460  318  48    52   o                    11   35.5                                                                             20.9                                                                             10.2                                                                             1.4                                                                             0.9                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 470  320  84    49   o                    12   35.6                                                                             20.9                                                                             10.3                                                                             0.8                                                                             0.3                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 410  305  65    83   o                    13   35.3                                                                             20.4                                                                             10.9                                                                             1.6                                                                             0.2                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 500  312  50    53   o                    14   35.1                                                                             20.5                                                                             10.1                                                                             1.9                                                                             0.3                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 550  321  47    39   o                    15   32.8                                                                             17.5                                                                             15.0                                                                             1.2                                                                             1.0                                                                             balance                                                                            --                                                                              --                                                                              0.07                                                                             0.08                                                                             0.03                                                                             590  372  83    64   o                    16   35.1                                                                             27.5                                                                             10.3                                                                             1.2                                                                             0.4                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 420  310  78    73   o                    17   35.0                                                                             29.6                                                                             10.2                                                                             1.1                                                                             0.3                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 420  305  82    81   o                    18   35.4                                                                             30.8                                                                             10.5                                                                             1.2                                                                             0.3                                                                             balance                                                                            --                                                                              --                                                                              -- -- -- 415  305  81    92   o                    __________________________________________________________________________     o: good                                                                       x: bad bead configuration                                                

                                      TABLE 2                                     __________________________________________________________________________    No. of                                  Hardness                                                                           Hard-                                                                             PbO PbO+                     Alloys                                  at   ness                                                                              resis-                                                                            PbSO.sub.4                                                                         Powder              (Compa-                                 Ordi-                                                                              at  tance                                                                             resis-                                                                             Build               rative                                  nary High                                                                              mg/ tance                                                                              Char-               Exam-                                                                              Composition (weight %)             temp.                                                                              temp.                                                                             cm.sup.2                                                                          mg/cm.sup.2                                                                        acter-              ple) Cr Ni   Mo C Si                                                                              Fe   Co   W Mn Al                                                                              Ti Hv   Hv  per hr                                                                            per                                                                                istic               __________________________________________________________________________    1    28.5                                                                             --   -- 1.1                                                                             1.3                                                                             --   balance                                                                            4.2                                                                             -- --                                                                              -- 430  310 62  127  o                   2    27.1                                                                             20.3 10.2                                                                             1.2                                                                             0.3                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 390  308 135 162  o                   3    41.2                                                                             20.5 10.8                                                                             1.4                                                                             0.2                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 710  351 55   64  o                   4    35.4                                                                             10.8 10.3                                                                             1.1                                                                             0.3                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 760  329 152  97  o                   5    35.0                                                                             32.3 10.1                                                                             1.3                                                                             0.4                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 410  298 81  123  o                   6    35.8                                                                             20.3  5.2                                                                             1.2                                                                             0.3                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 380  289 105 280  o                   7    35.4                                                                             20.8 24.6                                                                             1.4                                                                             0.3                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 680  325 123 118  o                   8    35.5                                                                             20.1 10.9                                                                             1.1                                                                             2.0                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 460  305 204  89  o                   9    35.3                                                                             20.1 11.0                                                                             1.1                                                                             2.4                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 470  308 420 103  o                   10   32.6                                                                             20.5 10.5                                                                             0.5                                                                             0.2                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 360  268 68   78  o                   11   37.5                                                                             20.3 10.4                                                                             2.5                                                                             0.3                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 680  341 51  136  o                   12   31.1                                                                             21.5  3.0                                                                             1.3                                                                             0.4                                                                             balance                                                                            --   3.1                                                                             -- --                                                                              -- 410  302 92  235  o                   13   27.4                                                                             15.0  2.2                                                                             1.1                                                                             0.6                                                                             balance                                                                            --   6.3                                                                             0.35                                                                             3.8                                                                             0.012                                                                            475  325 609 133  x                   14   30.5                                                                             balance                                                                             9.1                                                                             2.2                                                                             0.7                                                                             1.5  --   --                                                                              -- --                                                                              -- 425  280 61  284  o                   15   35.3                                                                             31.6 10.3                                                                             1.3                                                                             0.4                                                                             balance                                                                            --   --                                                                              -- --                                                                              -- 410  302 80  106  o                   __________________________________________________________________________     o: good                                                                       x: bad bead configuration                                                

(1) Hardness test at an ordinary temperature

The alloys of the preferred embodiments and comparative examples weremelted in an electric furnace in argon gas atmosphere, and molded in amold made of graphite. The mold had a cavity having an inside diameterof 15 mm and a chiller at the bottom thereof. The molded bodies were cutinto 15 mmφ×10 mm to obtain test pieces.

The hardness was measured at a chilled potion of the test piece, namelythe portion contacted with the chiller, by using a Vickers hardnesstester (load: 20 kgf).

(2) Hardness test at a high temperature

The same test pieces obtained for the above hardness test at theordinary temperature were employed to measure Vickers hardness at 700°C. using a Vickers hardness tester for a high temperature application(load: 5 kgf).

(3) PbO resistance test

The alloys of the preferred embodiments and comparative examples weremelted in an electric furnace in argon gas atmosphere, and molded in aglass tube by suction. The glass tube had an inside diameter of 5 mmφ.The molded bodies were cut into 5 mmφ×20 mm to obtain test pieces. Thetest pieces were immersed into 30 g of PbO heated at 920° C. for an hourto measure the corrosion loss.

(4) PbO+PbSO₄ resistance test

The same test pieces obtained for the above PbO resistance test werealso employed in this test. The test pieces were immersed into a mixtureof PbO and PbSO₄ heated at 900° C. for one hour to measure corrosionloss. The mixture weighed 30 g, and consisted of 60% by weight of PbOand 40% by weight of PbSO₄.

(5) Powder buildup test

The alloys of the preferred embodiments and comparative examples werepowdered by an ordinary gas atomizing. The powders of 80 to 350 mesheswere built up on an base metal of austenite heat resistance steel"SUH35" (Japanese Industrial Standards) by using a plasma powder buildupapparatus in the conditions shown in Table 3. The base metal size was 50mm in width×100 mm in length×10 mm in thickness. The wettability and thebead configuration were measured to evaluate the powder building upcharacter.

                  TABLE 3                                                         ______________________________________                                        Shielding gas discharge                                                                             30 l/min                                                Plasma gas discharge   4 l/min                                                Welding current       130 A                                                   Welding speed         65 mm/min                                               Powder supply         30 g/min                                                ______________________________________                                    

(6) Durability test using an actual engine and its results

Sample No. 3 of the preferred embodiments and sample No. 1 of thecomparative examples were powdered by the ordinary gas atomizing. Thepowders of 80 to 350 meshes were built up on a valve preform of theaustenite heat resistance steel "SUH35" in a condition shown in Table 4.The valve preform had a valve head having a diameter of 43 mmφ. Afterthe buildup, the valve preforms built-up with the alloys of sample No. 3of the preferred embodiments and sample No. 1 of the comparativeexamples were finished to a predetermined shape of predetermineddiameters. The valves were thus molded. The {O} content was 750 ppm inthe alloy powders of sample No. 3 of the preferred embodiments.

                  TABLE 4                                                         ______________________________________                                        Shielding gas discharge                                                                             40 l/min                                                Plasma gas discharge  4 l/min                                                 Welding current       120 A                                                   Welding speed         300 mm/min                                              Powder supply         10 g/min                                                ______________________________________                                    

A 2.5 liter in-line 6-cylindered gasoline engine was prepared, and thevalves built-up with the alloys of sample No. 3 of the preferredembodiments and Sample No. 1 of the comparative examples were mounted onthe first to third cylinders of the engine and the fourth to sixthcylinders thereof, respectively. Then, a durability test using theactual engine was conducted in the conditions shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Engine speed         4200 rpm                                                 Engine load          Full                                                     Cooling water temp.  90° C.                                            Lubricating oil temp.                                                                              120° C.                                           Fuel                 Leaded gasoline                                          Duration             300 hours                                                ______________________________________                                    

The corrosion losses of the faces of the valves built-up with sample No.3 of the preferred embodiments and sample No. 1 of the comparativeexamples were 8 to 12 μm and 10 to 21 μm, respectively. The depths ofthe corrosion in the faces of the valves were 2.5 μm and 23 μm,respectively.

(Evaluation)

As shown in Table 1, the alloys of the preferred embodiments weresuperior in hardness at the high temperature. Namely, the Vickershardness at the ordinary temperature was from Hv 390 to Hv 590, and theVickers hardness at 700° C. was Hv 300 or more for all of the alloys ofthe preferred embodiments.

Further, the alloys of the preferred embodiments were superior in thecorrosion resistance. Namely, the corrosion losses when immersed intothe PbO heated at 920° C. and the corrosion losses when immersed intothe mixture of 60% by weight of PbO and 40% by weight of PbSO₄ heated at900° C. were 100 mg/cm² per hour or less for both cases.

Furthermore, there arose no problems on the buildup operability when thepowdered alloys of the preferred embodiments were built-up on the valvepreforms. The deposition rates were 90% or more, and there were nodefects in the buildup layers.

Moreover, the engine valves built-up with the powdered alloys of thepreferred embodiments were superior in the abrasion resistance, the PbOresistance and the PbSO₄ resistance, and had performance sufficientlysatisfying the severe operation conditions of the high performanceautomobile engine.

On the other hand, as shown in Table 2, when either one of the componentelements did not fall in the limiting ranges of the present invention,at least one of the abovementioned characteristics; i.e., the hardnessat the high temperature, the abrasion resistance, the PbO resistance andthe PbO+PbSO₄ resistance deteriorated. As can be seen from the resultsof the durability test using the actual engine, the valve built-up withSample No. 1 alloy of the comparative examples was inferior in theabrasion resistance and the corrosion resistance. Sample No. 3 alloy ofthe comparative examples, in which only the chromium content did notfall in the limiting ranges of the present invention (namely thechromium content was 41.2%) exhibited the characteristics no less thanthe preferred embodiments did, however, Sample No. 3 alloy wasundesirable from the viewpoint of practicality. This is because thehardness at the high temperature, the corrosion resistance (the PbOresistance and the PbO+PbSO₄ resistance) and so on were not improvedgreatly even when more than 40% of chromium was added in the alloy. Theupper limit of the chromium content was thus set to 40% by weight in thepresent invention.

What is claimed is:
 1. An alloy for building up a valve comprising; withrespect to the total weight of said alloy taken as 100%, 30 to 40% byweight of chromium; 15 to 31% by weight of nickel; 7 to 20% by weight ofmolybdenum; 0.7 to 2.2% by weight of carbon; 1.5% or less by weight ofsilicon; and balance of iron and inevitable impurities, wherein thecontents of manganese, aluminum and titanium contained in saidinevitable impurities are 0.1% or less by weight, respectively.
 2. Analloy for building up a valve comprising; with respect to the totalweight of said alloy taken as 100%, 30 to 40% by weight of chromium; 15to 31% by weight of nickel; 7 to 20% by weight of molybdenum; 0.7 to2.2% by weight of carbon; 1.5% or less by weight of silicon; and balanceof iron and inevitable impurities, wherein said alloy is free frommanganese, aluminum and titanium.